Heat oven to 400°F. Cut pastry into six equal rectangles; brush with tablespoon milk. Bake on lined sheet 12 to 15 minutes until puffed and golden.
Meanwhile, melt butter in large soup pot; cook onion, carrots, and celery 6 minutes. Stir in flour 1 minute; gradually add broth.
Add potatoes, chicken, salt, thyme, and pepper. Gently simmer 15 to 18 minutes until potatoes tender and chicken 165°F.
Add peas and half-and-half; gently heat 3 to 4 minutes until peas hot and soup lightly coats spoon.
Divide among bowls; top each with one pastry rectangle just before eating.